Hulk Hogan -- Threatening LAWSUIT Over Leaked Sex Tape Footage

Hulk Hogan's leaked sex tape did NOT catch the wrestler with his pants down -- at least figuratively -- because TMZ has learned, he's already got his lawyers on the counter-attack, threatening to SUE if the tape isn't taken off the Internet.

Gawker posted a minute-long clip of the sex tape on its website yesterday -- but sources close to Hogan tell TMZ, it won't be up for long ... because Hogan's currently drafting a nasty cease-and-desist letter to the website, demanding the footage be taken down.

Hogan's claiming the site has no right to post the footage -- and if it doesn't remove it, he'll sue.

Hulk's lawyer David R. Houston tells TMZ, "This tape was made secretly without Hulk's knowledge or permission. We are going to do everything in our power to figure out who did it. Anyone displaying the tape will be held accountable to the fullest extent of the law."

As we first reported ... the Hogan tape was being shopped around to the highest bidder beginning earlier this year. Screen grabs hit the Internet in April, but video only surfaced yesterday.

Gawker says it obtained the tape from an anonymous source -- who gave the footage to the site for free.